Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses

Accepted Manuscript Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses

H. Wayne Nesbitt, Cedrick O'Shaughnessy, Grant S. Henderson, G. Michael Bancroft, Daniel R. Neuville PII: DOI: Reference:

S0009-2541(18)30594-1 https://doi.org/10.1016/j.chemgeo.2018.12.009 CHEMGE 18996

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

17 September 2018 1 December 2018 9 December 2018

Please cite this article as: H. Wayne Nesbitt, Cedrick O'Shaughnessy, Grant S. Henderson, G. Michael Bancroft, Daniel R. Neuville , Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses. Chemge (2018), https://doi.org/10.1016/ j.chemgeo.2018.12.009

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ACCEPTED MANUSCRIPT Factors affecting line shapes and intensities of Q3 and Q4 Raman bands of Cs silicate glasses H. Wayne Nesbitt1, Cedrick O’Shaughnessy2, Grant S. Henderson2, G. Michael Bancroft3, Daniel R. Neuville4

Dept. of Earth Sciences, University of Toronto, Ontario Canada

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Dept. of Chemistry, Univ. of Western Ontario, Ontario Canada

Géomatériaux, CNRS-IPGP, 1 rue Jussieu, Paris 75005, France

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Dept. of Earth Sciences, Univ. of Western Ontario, Ontario Canada

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Keywords: Raman spectra of Cs-silicate glasses, Q species abundances, Raman line shapes, Raman Linewidths, Lorentzian line shapes

E-mail: [email protected] 1

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ACCEPTED MANUSCRIPT ABSTRACT The Raman spectra of glasses containing 0 to 30 mol% Cs2O have been fit successfully with line shapes of dominantly Lorentzian character for the Q3 species, allowing quantification of Q3 and Q4 species intensities (Q represents a Si tetrahedron and the superscript indicates the

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number of bridging oxygen atoms, BOs, bonded to Si.) The intensity of the Q4 species A1

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symmetric stretch is exceptionally weak in vitreous silica (v-SiO2) but it increases dramatically

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with addition of small amounts of Cs2O to the glass. We propose that Cs, where in close proximity to BO of Q4 species, promotes the polarizability of Q4 tetrahedra and these primed Q4 species (Q4-

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p) produce a strong Q4 signal. There are, therefore, two variants of the Q4 species, a Q4-p species

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which produces a strong signal, and an unprimed species (Q4-u) which yields a very weak signal. The increase in the abundance of the primed Q4 species (Q4-p) can be modelled as a function of

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alkali content using a simple, upper-bounded growth model:

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XQ4-p = (1 – e-kx)

where XQ4-p is the fraction of polarizable Q4 species, k is a constant and x is the mol% counter

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oxide in the glass (here Cs2O). Comparison of calculated XQ4-p values with experimental results

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indicates that its cross section is similar to that of the Q3 species. There is no evidence for a ~1050 cm-1 band in the 5 mol% Cs2O glass spectrum but in the

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30 mol% Cs2O glass spectrum about 11% of spectral intensity is observed at about this frequency. The intensity likely results from development of asymmetry on the Q3 band, which increases with Cs2O content of the glass. The asymmetry results from weakened Si-O force constants of some Q3 tetrahedra due to charge transfer via Cs-BO bonds. As evidence, Si 2p and O 1s X-ray Photoelectron Spectroscopic (XPS) studies demonstrate that the electron density over Si and BO 3

ACCEPTED MANUSCRIPT atoms of Q4 species increases with Cs2O content. With charge transfer to tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing Si-O coulombic interactions, weakening Si-O force constants, and shifting the Q3 A1 symmetric stretch vibrational frequencies to lower values (e.g., from ~1100 cm-1 to ~1050 cm-1). The fraction of affected Q3 species increases

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with alkali content, as does the Q3 peak asymmetry. The Raman shifts of the Q4 species are also

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INTRODUCTION

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affected by increased Cs2O contents.

Alkali silicate glasses have been extensively studied by numerous techniques including

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Nuclear Magnetic Resonance (NMR) spectroscopy (e.g., Nesbitt et al., 2011; Maekawa et al.,

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1991; Stebbins, 1987), high resolution X-ray Photoelectron Spectroscopy or XPS (e.g., Nesbitt et al., 2015a; 2015b; Sawyer et al., 2015; 2012; Dalby et al., 2007) and Raman spectroscopy (e.g.,

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Bancroft et al., 2018; O’Shaughnessy et al., 2017; Hehlen and Neuville, 2015; Neuville, 2006,

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2005; Mysen and Frantz, 1994; 1993; Cooney and Sharma, 1990; McMillan, 1984; Matson et al., 1983; Seifert et al., 1982; Furukawa et al., 1981; Virgo et al., 1980; Brawer and White, 1975). The

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essential features of Raman spectra of alkali and alkaline earth silicate glasses were documented

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in the 1970s and early 1980s and by 1984 there was sufficient information for McMillan (1984) to summarize the Raman results (e.g., Matson et al., 1983; Furukawa et al., 1981; Virgo et al., 1980;

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Brawer and White, 1975). He noted that the A1 symmetric stretch of Q0 through Q4 species fell between ~800 cm-1 and ~1300 cm-1, and that the frequency of the A1 symmetric stretch for each Q species was similar regardless of the modifier cation (M) present (see also Nesbitt et al., 2017a). O’Shaughnessy et al. (2017) fit Cs-silicate glass Raman spectra using Gaussian line shapes as did most other Raman studies of alkali-silicate glasses. However, Bancroft et al. (2018) fit 4

ACCEPTED MANUSCRIPT spectra of 5 and 10 mol% Cs2O silicate glasses using line shapes of predominantly Lorentzian character for the Q3 band. The latter approach is consistent with theoretical considerations and results in simpler fits with fewer peaks being necessary to obtain good fits. The first goal of this communication is to test the findings of Bancroft et al. (2018) by fitting six Cs silicate glass spectra

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containing 5 to 30 mol% Cs2O, using predominantly Lorentzian line shapes for bands assigned to

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Q3 and Q2 species.

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Two ‘problems’ remain unresolved with respect to interpretation of Raman spectra of alkali and alkaline earth silicate glasses (e.g., Furukawa et al., 1981; McMillan et al., 1992; Neuville et

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al., 2014). One relates to the Q4 band intensity and its weak signal in vitreous silica (v-SiO2) and

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in glasses containing low alkali content. The other ambiguity relates to spectral intensity observed in the region between the Q3 and Q2 band maxima (here referred to as the 1050 cm-1 band). The

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two ‘problems’ are a major focus of this study.

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Q4 Band Intensity. Many publications in the last 35 years have noted the extremely weak Raman Q4 band at ~1200 cm-1 in v-SiO2 spectra (e.g., Bancroft et al., 2018; Nesbitt et al., 2017a;

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Neuville et al., 2014; Matson et al., 1983; Furukawa et al., 1981; Brawer and White, 1975).

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Sarnthein et al. (1997) and Pasquarello et al. (1998) applied first principles considerations to the Raman spectrum of v-SiO2 and identified the ~1200 cm-1 band to be an A1 symmetric stretch, and

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it has been interpreted as such subsequently (e.g., Bancroft et al., 2018; Le Losq and Neuville, 2017; Neuville et al., 2014; Rossano and Mysen, 2012; Neuville, 2006; McMillan, 1984; Le Losq et al., 2017). There is also a weak band at ~1060 cm-1 (Fig. 1) which Sarnthein et al. (1997) and Pasquarello et al. (1998) assigned as a stretching T2 vibrational mode. It should not be confused with the ‘1050’ band discussed subsequently. 5

ACCEPTED MANUSCRIPT Raman spectra of glasses with low alkali oxide content (e.g., <15 mol% M2O) reveal a very strong Q3 peak at ~1100 cm-1 and a weaker high frequency shoulder at ~1200 cm-1 with the latter identified as the Q4 band (e.g., Bancroft et al., 2018; Le Losq and Neuville, 2017; Nesbitt et al., 2017a; Neuville et al., 2014; Koroleva et al., 2013; Neuville, 2006; McMillan, 1984). The Q4

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intensity in 5 mol% Cs2O silicate glass, for example, should constitute ~85% of all Q species but

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is instead ~35%, as shown subsequently. A major ambiguity consequently remains concerning Q4

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intensities and it is here addressed.

The ‘1050’ Band Intensity. The second problem concerns spectral intensity observed

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between the Q3 and Q2 band maxima of alkali and alkaline earth silicate glasses. It is here referred

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to as the 1050 cm-1 band. Mysen et al. (1982) assigned the band to a Si-BO (or Si-O0) vibrational mode and Mysen and Frantz (1994; 1993) proposed that it was derived from vibrations of most or

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all Q species containing BOs. Kamitsos and Risen (1984) challenged the likelihood of a distinct

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1050 cm-1 band by fitting a 100% Lorentzian line shape to the Q3 band of a ~17 mol% Na2O glass. They concluded that no 1050 cm-1 band existed. Bancroft et al. (2018) similarly noted that there

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was no detectable 1050 cm-1 ‘band’ in 5 mol% Cs2O or K2O silicate glasses, both of which contain

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abundant Q4 and Q3 species. Clearly the Si-O0 vibrational mode of Mysen and Frantz (1994; 1993) does not originate with these Q species and it cannot originate with Q2 and Q1 species because they

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are effectively absent from the 5 mol% Cs and K glasses. Moreover, Bancroft et al. (2018) and McMillan et al. (1992) noted that the 1050 cm-1 band increased in intensity with Cs, K and Na content, making it highly unlikely that the intensity at 1050 cm-1 is derived from Si-BO vibrations (i.e., Si-O0) because BO decreases in glasses of increasing alkali content. McMillan et al. (1992) also emphasized that the intensity of the 1060 cm-1 band in vitreous silica (v-SiO2) was much too 6

ACCEPTED MANUSCRIPT weak to account for the 1050 cm-1 band intensity in glasses containing alkali oxides. From these observations we conclude that the 1050 cm-1 band is unlikely to result from Si-O0 vibrational modes on three counts: (1) high frequency Raman signals (800-1300 cm-1) are derived from vibration of the entire SiO4 molecule and these generally cannot be parsed and assigned to

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individual Si-BO bonds (Furukawa et al., 1981); (2) there is no 1050 cm-1 band intensity in 5 mol%

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Cs and K silicate glasses where Q4 and Q3 species are abundant and Q2 and Q1 are effectively

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absent so that a Si-O0 signal does not arise from any Q species containing BO; (3) the intensity of the 1050 cm-1 band increases with Cs, K, and Na content of glasses, indicating that its origin is

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unrelated to Si-BO vibrations.

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McMillan et al. (1992) suggested the 1050 cm-1 band “represents an A1 symmetric stretch vibration for a range of Q3 groups in a distribution of structural environments”, with some Q3 peaks

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being located at a somewhat lower frequency than the main Q3 peak; that is, they proposed that

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the Q3 band was asymmetric. O’Shaughnessy et al. (2017) proposed that the Q3 species signal was centered at ~1100 cm-1 but where Cs was bonded to a BO, some Q3 intensity was shifted to ~1050

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cm-1 and their explanation is consistent with that of McMillan et al. (1992). We expand on the

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ideas proposed by McMillan et al. (1992), O’Shaughnessy et al. (2017) and Bancroft et al., (2018)

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and address local electronic properties that affect frequencies of the Q3 band. METHODS AND PROCEDURES

The high resolution spectrum of SiO2 glass is that of Suprasil, taken under the same conditions described below (Fig. 1). The spectrum shown was limited to the range of 750-1400 cm-1 with an acquisition time of 200 s per scan. In total, 380 scans were acquired and summed. A second lower resolution spectrum was collected (not shown) over the range 16-1400 cm-1 to 7

ACCEPTED MANUSCRIPT evaluate the relative intensity of the high frequency bands. The Raman spectra of the 5 to 30 mol% Cs silicate glasses interpreted here (Figs. 2 and 3) are those reported by O’Shaughnessy et al. (2017) and Bancroft et al. (2018). Glasses were prepared from oxide components as reported by O’Shaughnessy et al. (2017). They were crushed

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in a mortar, melted and quenched repeatedly until all glasses were transparent with no observable

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defects or crystals. Raman spectra were collected at the Raman Laboratory of the Institut de

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Physique du Globe de Paris (IPGP) using a T64000 Jobin-Yvon triple grating Raman spectrometer equipped with a confocal system, a 1024 CCD detector cooled by liquid nitrogen and an Olympus

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microscope. A Coherent 70-C5 Ar+ laser was used, with an excitation wavelength of 488.01 nm

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and laser power of 100mW on the sample. Scans were taken in triple-subtractive mode which averages three spectra per sample. Resolution is better than 0.5 cm−1.

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CasaXPS software was employed to fit all Raman spectra using linear backgrounds and

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pseudo-Voigt (sum function) line shapes (Jain et al., 2018; Hesse et al., 2007). The Q3 and Q2 bands were fit with peaks of varying Lorentzian character and line width (full width at half-

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maximum or FWHM). In the absence of additional information we followed Mysen et al. (1982)

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and Bancroft et al. (2018) and employed 100% Gaussian line shapes to fit the Q4 bands. The Goodness of fit was determined by Root Mean Square (RMS) or χ2 parameters. Residuals were

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calculated for the fits to the 5 mol% Cs2O glass (Figs. 2, 3) as the percentage change in counts relative to the experimental data (i.e., Residuals in % = 100*(fit-expt.)/expt.). INTERPRETATION OF SPECTRA v-SiO2 Q4 bands The high frequency region of v-SiO2 Raman spectrum (Fig. 1) reveals a band at ~1060 cm8

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and one at ~1200 cm-1. Each contributes ~1% to the total v-SiO2 Raman intensity between 170

cm-1 and 1400 cm-1 (i.e., total intensity is the summed between these frequencies). The 1060 cm-1 band represents an asymmetric (T2) stretch and the higher frequency band a symmetric (A1) stretch (Sarnthein et al., 1997; Pasquarello et al., 1998). The best fit to the spectrum was obtained using

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four unconstrained 100% Gaussian peaks (Fig. 1a, residuals in Fig. 1b). The two peaks used to fit

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the 1060 cm-1 band have different areas, implying that the band is somewhat asymmetric. In

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contrast, areas and FWHM of the two peaks used to fit the 1200 cm-1 band are similar, demonstrating the near symmetric shape of the 1200 cm-1 band. Although symmetric, the 1200 cmband is too broad (flat-topped as noted by Mysen et al., 1982) to conform to either a pure Gaussian

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or Lorentzian distribution (Haken and Wolf, 1996). The two peaks used to fit the 1060 and 1200 bands are employed solely to obtain a good fit and are not meant to imply that there are two

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distinct contributions to each band, although Seifert et al. (1982) offer one (see also Neuville et

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al., 2014).

The unusually broad nature of the 1200 cm-1 band may relate to local structural variations

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in the glass. Whereas Si-O bond lengths and O-Si-O bond angles change little in crystals, glasses

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and melts (e.g., Henderson, 2005; Navrotsky et. al., 1985), the inter-tetrahedral angle () and the torsional angles (1 and 2) may vary (Hehlen et al., 2017; Henderson et al., 2006), thereby

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affecting tetrahedral site potential energies and associated ground states. Where  and  values take on a multitude of values, a multitude of ground states are produced and sampled during Raman excitation, which should produce an envelope of peaks (i.e., the Raman band) that reflects the distribution of  and  values in the glass. The Cs2O-SiO2 glass spectra 9

ACCEPTED MANUSCRIPT 5 mol% Cs2O spectrum. The 5 mol% Cs2O glass spectrum was initially fit using two peaks, one assigned to Q3 and one to Q4 (Fig. 2a, Table 1). The resulting Q4 peak, with FWHM equal 88.5 cm-1 (Table 1), is very broad and the residuals describe a sinusoidal pattern (Fig. 2b), indicating the fit can be improved by addition of another peak. A third peak yielded a better fit

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(Fig. 2c, 2d). The spectrum was fit a third time with the two Q4 peaks constrained in area so that

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Q4-1:Q4-2 = 1.0:0.908, which is the ratio obtained for the two peaks in the fit to the 1200 cm-1

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band of v-SiO2 (Fig. 1a). The result (Fig. 2e) is statistically indistinguishable from the fit of Figure 2c. The residuals (Fig. 2f), however, indicate a poor fit in the low frequency region. A fourth peak

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was introduced but was constrained to the same shape and width as the Q3-1 peak. The fit (Fig.

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2g) is excellent as indicated by the residuals (Fig. 2h). An explanation for the fourth peak is provided in the Discussion. The best fit was obtained with a 95% Lorentzian line shape. Fit

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parameters are listed in Table 1 and they are remarkably similar to those obtained by Bancroft et

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al. (2018). The similarity of fit parameters and peak areas derived from the two fits is an indication of the uncertainties associated with the fits.

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10 mol% Cs2O spectrum. The 10 mol% Cs2O spectrum is illustrated in Figure 3a.

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Although not shown, it was fit initially using a single 100% Gaussian peak to represent the Q4 species and two dominantly Lorentzian shaped peaks to fit the remainder of the spectrum (Q3 and

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all other contributions). An additional low frequency peak was required to obtain a reasonable fit. The fit parameters are provided in Table 1 (10 mol% Cs2O, Q4 unconstrained). The Q4 peak is very broad (83.5 cm-1) suggesting an inadequate fit (Bancroft et al., 2018). The spectrum was fit again using two Q4 peaks where the Q4-1:Q4-2 peak areas were constrained to 1.0:0.908, as in v-SiO2. The fit is shown in Figure 3a and fit parameters are listed in Table 1. The results are similar to 10

ACCEPTED MANUSCRIPT those obtained by Bancroft et al. (2018). The interpretation of the peak centered at 982 cm-1 is not obvious. Its Raman shift is within the frequency range quoted for a Q2 species (McMillan, 1984) but it may be a Q3 contribution as addressed in the Discussion. 15 mol% Cs2O spectrum. The spectrum was initially fit using one unconstrained Gaussian

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peak to represent the Q4 species and three peaks of 85% Lorentzian character to represent all other

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contributions. Although not plotted, the fit parameters are listed in Table 2 (15% Cs2O, Q4

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unconstrained). The Q4 peak was considered too broad at 79.1 cm-1 to be reasonable, and another fit was attempted. The best fit was obtained with two Gaussian peaks representing Q4 species

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constrained to the area ratio obtained for v-SiO2 (1.0:0.908) and with the other peaks set to 80%

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Lorentzian character (Fig. 3b). Fit parameters are listed in Table 2. The peak labelled Q3-3? is located at ~966 cm-1, which is within the frequency range of the Q2 species (McMillan, 1984). The

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assignment is addressed in the Discussion.

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20 mol% Cs2O spectrum. The fit to the 20 mol% spectrum (Fig. 3c) includes the same number of peaks as used to fit to the 15 mol% spectrum, although the fit parameters differ

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somewhat (Table 2). The Q4-1:Q4-2 area ratio was constrained to a 1.0:0.908 and the resulting fit

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has the two Q4 peaks almost superimposed (Fig. 3c), indicating that one peak should suffice to represent Q4 species. The result of using one unconstrained Q4 peak is illustrated in Figure 3d. The

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FWHM of the Q4 peak is not excessively broad indicating that one Gaussian peak is sufficient to account for the Q4 contribution. The lowest frequency peak is very weak at 1.3-1.4% but its Raman shift at ~942 is well within the frequency range of the Q2 species (McMillan, 1984). On this basis it is interpreted as a Q2 species band. The ~966 and ~968 cm-1 bands of the 15 mol% Cs2O glass (Fig. 3b, Table 2) may also represent a Q2 signal but as previously indicated the assignment is 11

ACCEPTED MANUSCRIPT uncertain. 25 and 30 mol% Cs2O spectra. The fit of Figure 3d was used as template to fit the 25 and 30 mol% Cs2O spectra (Figs. 3e, 3f). The resulting fits were poor on the low frequency side of the Q3 peak and an additional peak (labelled Q3-3) situated between the Q2 and Q3-2 peaks was

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required to obtain a good fit. There are no spectral features to constrain the width of the Q3-2 or

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Q3-3 peaks and they were set to the same FWHM and peak shape as the Q3-1 peak. The explanation

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for assigning the ‘Q3-3’ peak as a Q3 contribution is provided in the Discussion. The best fit to the 30 mol% Cs2O spectrum is illustrated in Figure 3f and fit parameters are listed in Table 2. To test

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the sensitivity of the fit to line shape, the 30 mol% spectrum was refit with the Q3 Lorentzian

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component decreased from 80% to 75%. The Q4 species abundance was most affected by change of line shape, increasing from ~8% to ~15% (see last two entries in Table 2).

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DISCUSSION

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Q4 and Q3 Peak Parameters

The 1200 cm-1 Q4 band of v-SiO2 is effectively symmetric but it displays an unusually

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broad, flat top which cannot be accurately fit with either a Gaussian or a Lorentzian line shape.

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The Q4 band intensity of v-SiO2 and siliceous Cs2O glasses may be evaluated using one or two Gaussian peaks to fit the band in that both yield intensities within ~5 mol% of each other (Tables

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1 and 2). The two-peak fits, however, yield lower RMS values. The FWHM of each peak of the two-peak fits is less than ~75-80 cm-1 which is here considered an upper limit for Q4 band widths (see also Bancroft et al., 2018). The one-peak fit yields FWHM greater than 75 cm-1 for glasses containing less than 20 mol% Cs2O but at greater Cs2O concentrations, a one-peak fit seems more appropriate in that a two-peak fit has the two peaks effectively superimposed (e.g., Fig. 3c). The 12

ACCEPTED MANUSCRIPT use of two peaks to fit the Q4 band does not imply that two distinct Q4 signals are present, as explained in a previous section entitled ‘v-SiO2 Q4 bands’. The fitted Q3 peak shapes are strongly Lorentzian and are consistent with the fits of Bancroft et al. (2018) for 5 and 10 mol% Cs2O glasses. The Lorentzian dominance of shape extends

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to Q3 bands of glasses containing 30 mole % Cs2O. Importantly, there is no need to introduce a

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major peak at ~1050 cm-1; it likely is an artifact of employing 100% Gaussian peak shapes (e.g.,

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O’Shaughnessy et al., 2017; Le Losq and Neuville, 2017). Sensitivity of all fits to variations in line shape was tested by varying the Lorentzian component in 1% increments, using the pseudo-

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Voigt sum function. The Lorentzian percentages quoted in Tables 1 and 2 may vary up to ±5%

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without appreciable degradation of the fits. There is a trend toward decreased Lorentzian component with increased Cs2O content, decreasing from 95±5% at 5 mol% Cs2O to 80±5% at 30

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mol% Cs2O. The explanation is uncertain but it may be related to production of NBO. New

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environments (local structures) are generated by formation of NBOs, resulting in a greater range of distinct local structures, leading to greater variation in tetrahedral site potentials and their ground

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states, which may increase the Gaussian component of the Q3 band shape.

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The Q3 FWHM is 40 cm-1 in the 5 mol% glass (as obtained by Bancroft et al., 2018) and increases to ~50-56 cm-1 for the 25-30 mol% glasses. The increase in line width with Cs2O content

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may result from creation of a greater range of local structures resulting from the presence of the modifier cation, as proposed for changes to line shape. Q4 species intensities The Q4 intensities listed in Tables 1and 2 are plotted on Figure 4a and they describe an arcuate trend with respect to glass composition. The trend is unexpected in that the most siliceous 13

ACCEPTED MANUSCRIPT glass (5 mol% Cs2O) should contain mostly Q4 species (~90 mol%). Instead, the Q4 band is less intense than the Q3 band and an investigation follows. Expected Q4 Abundances. Using the approximation that each Cs atom gives rise to one NBO, and assuming that only Q3 and Q4 species are present (i.e., no Q2 species) in low Cs glasses,

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then each NBO produced must give rise to a Q3 species. At 0 mol% Cs2O (pure SiO2), Q4 = 100%

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(Fig. 4a). Addition of Cs2O converts some BO to NBO (and Q4 to Q3 species) so that Q4 mol%

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decreases from 100% in v-SiO2 to zero mol% at 33.3 mol% Cs2O (Fig. 4a, dashed line). The above assumptions should yield accurate Q4 and Q3 estimates between 0 and ~20-25 mol% Cs2O but the

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estimates are less accurate for Cs2O > 20-25 mol% due to polymerization and disproportionation

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reactions (Nesbitt et al., 2011; Stebbins, 1987). The Q4 percentages derived from the fits (Fig. 4a) approximate the calculated Q4 percentages for Cs2O > 20 mol%.

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Polarizability of Q4 species. The Q4 and Q0 species both have tetrahedral symmetry and

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the A1 symmetric stretch is Raman allowed in both species. Somewhat surprisingly, the A1 symmetric stretch of the Q0 species is strong (e.g., Voronko et al., 2006) whereas the A1 symmetric

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stretch of Q4 is very weak in v-SiO2 (Matson et al., 1983; Furukawa et al., 1981; Brawer and White,

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1975). Clearly, the weakness of the Q4 signal is not related to molecular symmetry but to some other property. Furukawa et al. (1981) note that “Raman scattering intensity is proportional to the

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change of polarizability during execution of a normal vibration.” They also note that a change in polarizability is determined primarily by the change in Si-O bond length during excitation by an incident photon. Addition of only 5 mol% Cs2O to the glass yields much stronger Q4 intensity and approaches the Q3 band intensity (e.g., Fig. 2a). The Q4 band is still more intense in 10 mol% Cs2O glass (Table 1, Figs. 2g and 3a) making it apparent that the Q4 species intensity is promoted by 14

ACCEPTED MANUSCRIPT addition of Cs to the glass. Perhaps Cs relaxes the SiO4 structure around the cation, allowing greater Si-O normal vibrational excursions during excitation by photons. If so, there should be a relationship between Cs coordination number (CN) and Q4 intensity. For Cs glasses, Minami et al. (2014) and Kaneko et al. (2017) obtained Cs coordination

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numbers (to oxygen) of 11 and 10 respectively. Of the oxygen atoms, one must be NBO and at

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very low Cs concentrations, the others likely are BOs. If, through close approach of Cs to the BOs,

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the alkali were to enhance the polarizability of the associated Q4 species, then up to 9 Q4 molecules would be primed to contribute to the Q4 band intensity per Cs atom added to the glass. The dotted

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curve of Figure 4a illustrates the trend and at low Cs2O concentrations the Q4 Raman intensity

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increases in approximate proportion to the Cs coordination number (CN) where the slope ~ the Cs CN. The slope of the trend is consistent with Cs being responsible for priming Q4 species. These

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primed Q4 tetrahedra are referred to subsequently as Q4-p species.

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A Q4-p growth model. In v-SiO2, where Cs ions are absent, the concentration of Q4-p species is zero. In Cs-rich glasses Q4-p will have a maximum value equal to the total number of

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Q4 species in the glass. The fraction of primed Q4 species (XQ4-p) therefore increases from 0.0 in

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v-SiO2 to a maximum of 1.0 in basic melts where all Q4 species are primed. Growth to a maximum value is a characteristic of ‘simple bounded growth’ where the growth in Q4-p (per mol of Cs) is

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proportional to the concentration of unaffected or ‘unprimed’ Q4 species present (Q4-u), as for radionuclide decay and growth of daughter products. The equation is (e.g., Varberg and Flemming, 1991): XQ4-p = L(1 – e-kx) + X*e-kx

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where XQ4-p is the fraction of primed Q4 species, L is the maximum value of XQ4-p (=1.0), x is the 15

ACCEPTED MANUSCRIPT independent variable (i.e., mol% of alkali oxide) and X* is the fraction of ‘polarizable’ Q4 at x = 0 (i.e., XQ4-p = 0.019 in v-SiO2) and as an approximation is set to 0.0 subsequently. The constant ‘k’ determines the rate of growth of primed Q4 species. Total Q4 intensity. The cross section of the ‘unprimed’ Q4 species (Q4-u) has been

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calculated at ~0.019 (Furukawa et al., 1981). The cross section of primed Q4 species (Q4-p) must

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be much greater, as evident by the presence of the strong Q4 signal in Figure 2a, in which Q3 and

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Q4 species intensities are of the same order. The cross sections of the Q1 to Q3 species range from

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1.4 to 0.9 (Furukawa et al., 1981, their Fig. 9) and taking Q3 as the best analogue the Q4-p cross section should be ~1. The intensity (I) of a band is the product of its cross section () and its

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concentration (X) where X is mole fraction: I = X

(2)

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There are two contributions to the Q4 intensity, Q4-p and Q4-u, so the total Q4 intensity (IQ4) is: IQ4 = Q4-pXQ4-p + Q4-uXQ4-u

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(3)

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but from mass balance considerations: XQ4-u = 1 – XQ4-p

(4)

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Substitution of Equation 4 into Equation 3 yields: IQ4 = Q4-pXQ4-p + Q4-u(1-XQ4-p)

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(5)

and substitution of Equation 1 into Equation 5 yields: IQ4 = Q4-p[L(1 – e-kx) + X*e-kx] + Q4-u(1-[ L(1 – e-kx) + X*e-kx])

(6)

With Q4-u ~ 0.0 (i.e., 0.019, Furukawa et al., 1981) and X* = 0.0 at x = 0, Equation 6 reduces to: IQ4 = Q4-p[L(1 – e-kx)]

(7)

where L = 1.0 (see above). The remaining unknowns are Q4-p and ‘k’. The solid curve of Figure 16

ACCEPTED MANUSCRIPT 4a was calculated with Q4-p = 1.1 and k = -0.085 and it reproduces reasonably the Q4 intensities. The initial slope of the solid curve trend of Figure 4a is almost coincident with the dotted line indicating that at low Cs2O concentrations the IQ4 intensity is consistent with a Cs coordination number of ~10. The two trends diverge for Cs2O > ~2-3 mol% and probably results from non-

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random distribution of Cs in the glasses (e.g., modified random network, Greaves et al., 1981, Le

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Losq et al., 2017) and from an increased number of NBO atoms coordinated to Cs (e.g., Du and

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Cormack, 2004).

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Polarization of a symmetric molecule by an ion operates at very short range, with the energy of interaction decreasing as the fourth power of the distance between their centers (Huheey

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et al., 1993, p. 299; Moellwyn-Hughes, 1961, p. 308). It is therefore feasible for a cation to change the polarizability of Q4 molecules, with BOs located within its first coordination sphere, while

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having no effect on the polarizability of more distant Q4 species. We suggest that the cation

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changes polarizability of the Q4 species by distorting molecular orbitals over bridging oxygen

(e.g., Figs 2 and 3).

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atoms located within the 1st coordination sphere of the cation, leading to enhanced Q4 intensities

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Q3 asymmetry and the 1050 cm-1 band Background. McMillan et al. (1992) fit two Q3 bands to a K-disilicate spectrum at ~1104

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cm-1 and at 1087 cm-1, with the latter band being exceptionally broad, at ~132 cm-1. Use of Lorentzian-dominated peaks reduces dramatically the need for a broad peak at 1087 cm-1 (e.g., Bancroft et al., 2018 and Fig. 3f). The problem addressed by McMillan et al. (1992) nevertheless remains – there is unexplained intensity in the region 1000-1100 cm-1 and most studies have introduced a band centered at ~1050 cm-1 to account for it (e.g., Bancroft et al., 2018; Le Losq and 17

ACCEPTED MANUSCRIPT Neuville, 2017; Koroleva et al., 2013; Neuville, 2006; Mysen and Frantz, 1993; McMillan, 1984; Matson et al., 1983; Furukawa et al., 1981). As noted in the Introduction, the band has been variously interpreted (e.g., Neuville et al., 2014; Frantz and Mysen, 1995; Mysen and Frantz, 1994; McMillan et al., 1992). The band is, however, absent from the 5 mol% Cs, Rb and K silicate glasses

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(Bancroft et al., 2018; Matson et al., 1983) leading Bancroft et al. (2018) to conclude that addition

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of these alkalis quenched the 1050 cm-1 band (interpreted as a T2 asymmetric stretch). With the

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uncertainty in the interpretation, we turn to basic vibrational theory to constrain possibilities. Controls on Q3 Vibrational frequencies. Hooke’s Law relates the frequency of a

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molecular vibration () to the reduced mass () of vibrating atoms and the force constant (f)

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according to (Moellwyn-Hughes, 1961, p. 402, 509):

 = (1/2)(f/)1/2

(8)

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The reduced mass of a regular tetrahedral, pentatomic molecule is equal to the mass of the corner

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atom (Moellwyn-Hughes, 1961, p. 509), and it should be effectively constant (at ~16 atomic mass

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units) for SiO4 in vitreous silica. Introduction of alkalis produces NBO, changing the symmetry of the SiO4 moiety and introducing the possibility that the counter cation may affect vibrational

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properties through changes to the reduced mass. In all alkali glasses the average Raman shift for

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the Q3 species is 1098 cm-1 and it is largely independent of the type and concentration of counter cation (see Fig. 1b of Nesbitt et al., 2017a). From this observation it is concluded that the modifier cation does not contribute appreciably to the reduced mass or to the frequency of the Q3 A1 symmetric stretch. This leaves the force constant (Eq. 8) as the only variable to affect appreciably the frequency of the A1 symmetric stretch of the Q3 species. X-ray photoelectron spectroscopic (XPS) results (Figs. 5a to 5c) show that the energy with 18

ACCEPTED MANUSCRIPT which the O 1s and Si 2p electrons (of SiO4) are bound to their respective nuclei (i.e., binding energy or BE) decreases as alkali content increases (Nesbitt et al., 2017a; 2017b). The decreased BE results from an increase in valence electron density on each atom. The increase in electron density (hence decrease in BE) arises mostly from transfer of ‘s’ electrons from alkali atoms to

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molecular orbitals of the tetrahedra (Nesbitt et al., 2017a; 2017b; Briggs and Seah, 1990). The

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electron density is then redistributed over all atoms of the tetrahedron via MOs (Nesbitt et al.,

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2017a; 2017b) but as apparent from the slopes of the trends in Figures 5a to 5c, it accumulates more on Si than on BO or NBO. Preferential accumulation of charge on Si weakens all Si-O

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coulombic interactions and by Equation 8, weakens Si-O force constants thereby decreasing SiO4

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vibrational frequency of the A1 symmetric stretch (Nesbitt et al., 2017a; 2017b). The relationship is sufficiently robust that Qn species Raman shifts can be forecast from the Si 2p BE shifts (Fig.

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5e). Moreover, both Raman shift and Si 2p BEs, are approximately linearly dependent on alkali

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oxide composition (Figs. 5c, 5d) from which is derived: Q species A1 Symmetric Stretch = 100x(-0.054)x(mol% alkali oxide) + 1218

(9)

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where -0.054 is the slope derived from the fit to the Si 2p data of Figure 5c. We conclude that

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electron density over Si tetrahedra controls the A1 symmetric stretch frequencies through changes to the Si-O force constants.

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Electron transfer to SiO4 tetrahedra. Addition of network modifiers (e.g., alkali oxides) to v-SiO2 results in destruction of Si-BO-Si linkages and generation of NBO which, from an ionic perspective, occurs through transfer of an alkali ‘s’ electron to a BO of the reactant Q4 species to produce a Q3 species (e.g., Henderson et al., 2006; Henderson, 2005). The creation of NBO by charge transfer from an alkali is here referred to as a first order charge transfer process because it 19

ACCEPTED MANUSCRIPT results in a dramatic increase in electron density on the Q3 tetrahedron produced. In response, there is a decrease in Q3 Si-O force constants and a corresponding decrease in the frequency of the A1 symmetric stretch of the product Q3 species compared with the reactant Q4 species (A1 symmetric stretch decreases by ~100 cm-1 as shown in Fig. 5e).

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There is also a second order charge transfer process. It affects electron density over

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tetrahedra to a lesser extent than the first order process. Alkali ions are coordinated to both NBO

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and BO atoms in glasses and melts. Alkali-BO bonds, although strongly ionic, display weak covalent character which results in some charge being transferred to BO atoms located within the

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alkali atom’s first coordination sphere (e.g., Nesbitt et al., 2017a,b; Du and Corrales, 2006; Tilocca

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and De Leeuw, 2006; Du and Cormack, 2004). The O 1s XPS results of Sawyer et al. (2012) provide the evidence for charge transfer via BO atoms. As shown in Figure 5f, the BEs of Si 2p

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electrons of Q4 and Q3 species decrease somewhat with increase in alkali content of the glasses,

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which as previously argued, results from increased electron density on Si of both Q species. The increased negative charge on the Si atom of Q4 tetrahedra can occur only through charge transfer

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via BO atoms with the charge then redistributed to all atoms of the tetrahedron. The increased

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negative charge of Si weakens the Si-BO force constant thus decreasing Q4 vibrational frequencies (Fig. 5f). Both first and second order charge transfer processes decrease Si-O force constants and

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the frequency of the A1 symmetric stretch of Q4 and Q3 species. The local environment determines in turn, the extent to which charge density changes on the Q species as proposed by McMillan et al. (1992). Q species Raman line shapes Alkali-BO Interactions. Addition of a small amount of Cs2O to v-SiO2 results in 20

ACCEPTED MANUSCRIPT formation of NBO and in conversion of a Q4 band to a Q3 band centered at ~1100 cm-1 (Fig. 2a). This first order electron transfer process produces a narrow, symmetric, predominantly Lorentzian Q3 band, as portrayed in Figure 6 (solid curve) and as observed in Figure 2a. Second order charge transfer processes affect peak shapes by producing asymmetry on the low frequency side of the

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(mostly) Lorentzian Q3 band. Once a Q3 band is produced by formation of NBO, continued

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increase of Cs2O may affect its Raman shift through the second order charge transfer process (i.e.,

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charge transfer via BO atoms to Q3 species), weakening the Q3 Si-O force constants and shifting individual Q3 peaks to somewhat lower frequency (Fig. 6, dashed curve and Nesbitt et al., 2017c;

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2015a). Band asymmetry therefore should increase with additional of Cs2O to the glass. The

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generation of asymmetry via the second order process should occur on all Q species containing BO.

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Qn,ijkl clustering. Another second order process may produce similar asymmetry and it

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relates to clustering of Qn,ijkl species (e.g., Olivier et al., 2001) and it operates through BO atoms. Glock et al. (1998) and Olivier et al. (2001) conducted

29

Si double quantum NMR studies and

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calculations on the efects of various types of Q species bonded to a central Qn species (i.e., Qn,ijkl

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clusters). The Qn,ijkl notation (Glock et al., 1998) indicates that a central Q species incorporating ‘n’ BOs, whereas i, j, k and l represent the type of Q species bonded (via BOs) to the central Q

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species. Experimental results indicate that the central Q3 peak shifts from -95 to -89 for the Q3,344 and Q3,333 clusters (Glock et al., 1998). Equivalent results were obtained for Q4,ijkl clusters in K and Na silicate glasses (Sen and Youngman, 2003; Olivier et al., 2001). The results are interpretable from the perspective of electron densities over the Q species. The electron density over a Q3 species is greater than that over Q4 species due to creation of an 21

ACCEPTED MANUSCRIPT NBO (a first order process). A central Q3 species (or any Qn species) bonded to four Q3 species (a Q3,3333 cluster) will sport greater electron density over its tetrahedron than will a central Q3 species bonded to four Q4 species (a Q3,4444 cluster) due primarily to negative charge being transferred to the central Q species via the shared BOs. Thus the effects on electron density of alkali-BO

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interactions and Qn,ijkl clustering should be similar, although the former is likely to have more

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effect on electron density because the alkali constitutes the second coordination sphere of BO

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atoms. As with alkali-BO interactions, Qn,ijkl clusters likely decreases frequencies of the A1 symmetric stretch of Q species, producing Q3 band asymmetry (Fig. 6).

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CONCLUSIONS

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Q4 Raman band properties

Fits to Cs2O silicate glass spectra ranging from 5 to 30 mol% Cs2O confirm the findings of

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Bancroft et al. (2018); Q3 bands are strongly Lorentzian in shape and display FWHM ranging from

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~40-55 cm-1. Our Q4 species findings re-enforce those of Mysen et al. (1982) in that two 100% Gaussian peaks best fit the A1 symmetric stretch of the Q4 species (Fig. 1). The band, although

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symmetric, is too broad to be of Gaussian or Lorentzian shape (or a combination of these shapes)

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and its breadth may be related to local structural variations in, for example, distribution of Si-OSi angles or torsional angles (Bancroft et al., 2018; Hehlen et al., 2017; Sen and Thorpe, 1977).

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The FWHM of the Q4 band decreases markedly from 5 to 30 mol% Cs2O glass (Tables 1 and 2) and approaches that of the Q3 linewidth. The Q4 Raman shifts also decrease noticeably from the 5 mol% spectrum to the 30 mol% spectrum. Estimates of Q4 abundances obtained using one and two Gaussian peaks are similar (within ±5% absolute) but FWHM of one-peak fits are unacceptably broad in highly siliceous glasses (> 80% SiO2). 22

ACCEPTED MANUSCRIPT The Q4 species intensity is weak in v-SiO2 but it increases dramatically with addition of alkali oxides to a glass (Bancroft et al., 2018; Matson et al., 1983). The implication is that the alkalis, and probably the alkaline earths, promote electronic polarizability of the Q4 species during excitation thus producing a more intense Q4 Raman signal. At low Cs2O concentrations the Q4

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Raman activity increases in approximate proportion to the coordination number of Cs. Q4 species

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are of two types, those with enhanced polarizability (Q4-p type) and those which are not enhanced

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(Q4-u). The former have a cross section similar to that of the Q3 cross section while the latter has a cross section 50- to 100-fold weaker.

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The Q3 and 1050 cm-1 band

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We propose that the intensity between the Q3 and Q2 band frequencies results from the A1 symmetric stretch of Q3 species. The Q3 band is asymmetric due to Cs-BO interactions (Nesbitt et

M

al., 2017a; 2017b; 2015a) or to Qn,ijkl species clustering (Olivier et al., 2001). The asymmetry

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results from weakened Si-O force constants which decrease the Q3 species A1 symmetric stretch to values less than 1100 cm-1. This also explains the increase in asymmetry with increased alkali

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oxide content (Figs 2, 3). The explanation is consistent with line shapes being affected by local

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environments surrounding Q3 species (McMillan et al., 1992). A simple approach to account for the asymmetry is to introduce additional (symmetric) peaks at lower frequency than the main Q 3

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peak (e.g., O’Shaughnessy et al. 2017; Le Losq and Neuville, 2017; Neuville, 2006). The peaks have no theoretical significance but they provide reasonable estimates of the total Q3 peak area. Implications for percolation channels The percolation channel model for alkali-silicate glasses proposed by Greaves et al. (1981) is widely accepted amongst the glass community, and has recently been suggested to be common 23

ACCEPTED MANUSCRIPT to all glass compositions (Le Losq et al., 2017). If these structures exist then we must accept that parts of the glass structure are more “organized” than others. Given that crystalline Raman peaks are narrow and ~100% Lorentzian in shape (Nesbitt et al., 2018), the broader peaks with greater Gaussian component commonly observed in glasses may be due in part to variations in local setting

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within glasses. We speculate that the variation in Lorentzian/Gaussian line shapes may be

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indicative of changes in the degree of ordering in the glass and, in particular, the development and

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extent of percolation channels. Indeed, the Lorentzian % assigned to the Q3 peaks was evaluated by trial and error, and additional studies, some of which are underway, are required to rationalize

AN

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the changes to Lorentzian- Gaussian peak shapes as a function of composition of the glasses.

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Virgo, D., Mysen, B.O., Kushiro, I., 1980. Anionic constitution of 1-atmosphere silicate melts: implications for the structure of igneous melts, Science, 208, 1371–1373. Voronko, Y.K., Sobol, A.A., Shukshin, V.E., 2006. Raman Spectra and Structure of Silicon– Oxygen Groups in Crystalline, Liquid, and Glassy Mg2SiO4. Inorg. Materials, 42, 981-988. 30

ACCEPTED MANUSCRIPT Figure Captions Fig. 1: (a) High frequency portion of the Raman spectrum of vitreous silica. Each band was fit with two 100% Gaussian peaks so as to obtain a good fit. The terms ‘1060 band’ and ‘1200 band’ are employed in the text (and in most of the literature) to refer to these bands. The

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‘1060 band’ is somewhat asymmetric where as the ‘1200 band’ is effectively symmetric.

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(b) The residuals plot where the residuals were calculated as the percentage difference

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between the fitted results and spectral data normalized to the spectral data. Fig. 2: Raman spectrum of the 5 mol% Cs2O glass with fits and residuals. The best fits to the

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spectra are illustrated by the thick solid curves which, for the most part, intersect the

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experimental data. Thinner solid curves represent the peaks fit to the spectrum. The fits are progressively better in the order 2a, 2c, 2e and 2g as apparent by the Root Mean Square

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(RMS) values and as shown by the associated residuals plots. The fits of 2a and 2c are

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unconstrained. The Q4 fitted peaks are constrained so that the Q4-1:Q4-2 area areas equal 1.0:0.908. Fit parameters are provided in Table 1. See text for additional details.

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Fig. 3: Raman spectra of 10, 15, 20, 25 and 30 mol% Cs2O glasses. The best fits to the spectra are

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illustrated by the thick solid curves which mostly intersect the experimental data. The thinner solid curves represent the fitted peaks. Figs. 3c and 3d use two and one Q4 peak to

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fit the data and RMS values demonstrate there is little to choose between the two fits. Fit parameters are given in Table 1 and are discussed in the text. Fig. 4: (a) Illustrates the relationship between composition and the mol% of Q4 species (both total and polarizable Q4 species). Solid circles represent the Q4-p mol% derived from fitting the spectra of Figures 2 and 3 (see Table 1). The capped bars indicate a ± 4% (absolute) range 31

ACCEPTED MANUSCRIPT in intensity and they are shown to aid assessment in variability of Q4 areas from different fits. The dashed line represents total Q4 mol% in the glasses calculated by employing the mass balance assumption that each Cs atom gives rise to a NBO atom in the glass and that only Q3 and Q4 species are present in the glass (see text for details). The dotted line

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represents the mol% of Q4-p species (polarizable Q4 species) calculated assuming that each

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Cs atom is coordinated to 1 NBO and 9 BO atoms (see text for details). (b) illustrates the

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relationship between the fraction of polarizable Q4 species (Q4-p) as a function of Cs2O mol% in the glass and the dotted line represents the fraction of polarizable Q4 assuming

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that each Cs atom is coordinated to 1 NBO and 9 BO atoms (see text for details).

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Fig. 5: (a) to (c) Plots of NBO 1s, BO 1s and Si 2p XPS peak maxima plotted against Na2O and K2O content of binary glasses with error bars representing ±0.2 eV (data from Nesbitt et

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al., 2011 and Sawyer et al., 2012). The straight lines are least squares fits to the data. The

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negative slopes of the trends in (a) to (c) demonstrate that electron densities over NBO, BO and Si atoms increase with alkali oxide content (i.e., BEs decrease), and that electron

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density over Si increases the most per mol of alkali oxide in the glass (slope is steepest).

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(d) relationship between and alkali oxide content of glasses (e) The relationship between Raman shift of Q species and Si 2p binding energies. The strong correlation indicates that

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Raman shifts and Si 2p BEs are controlled by the same property, primarily electron densities over Si atoms of tetrahedra. (f) Illustrates Si 2p BE shifts in Q4 and Q3 species as a function of K2O content of the glasses. The results demonstrate that electron densities on Si of Q4 and Q3 species increase with K2O content. With respect to Q4 species this can occur only through charge (electron) transfer via BO atoms to Si of the tetrahedra. 32

ACCEPTED MANUSCRIPT Fig. 6: Schematic illustrating a symmetric Lorentzian peak (solid curve) and an asymmetric peak (dashed curve) where both are normalized to the same maximum. The arrows indicate the effects of the first order electron transfer process, which gives rise to a symmetric, highly Lorentzian Q3 signal (i.e., charge transfer during creation of NBOs) and second order

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electron transfer processes which produces asymmetry in the Q3 signal (i.e., charge

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transfer to tetrahedra via BO atoms).

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ACCEPTED MANUSCRIPT Table 1: Fit parameters for v-SiO2, 5 and 10 mol% Cs2O glasses

5 mol% Cs2O glass

2a

Q4 unconstrained 5 mol% Cs2O glass

2c

Q4 unconstrained

cm-1

%Lor.1

cm-1

%

1060-1

1055

0

57.4

36.9

1060-2

1091

0

47.8

17.6

1200-1

1183

0

73.0

23.9

1200-2

1238

0

70.7

Q4-1

1164

0

88.5

35.0

Q3-1

1098

99

39.7

65.0

Q4-1

1189

0

73.1

16.2

Q4-2

1151

0

55.5

14.7

Q3-1

1099

99

40.3

69.1

Q4-1

1190

0

77.4

17.8

1152

0

57

16.2

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Area

21.7

Q4-2

by area

Q3-1

1098

95

40.2

64.2

Q3-2?

1019

95

40.2

1.8

Q4-1

1158

0

83.5

40.6

Q3-1

1097

90

41.3

55.0

Q3-2

1020

90

41.3

2.8

Q3-3?

985

90

41.3

1.7

Q4-1

1179

0

69.7

19.3

Q4-2

1145

0

58.9

17.5

Q3-1

1097

90

42.1

58.6

Q3-2

1003

90

42.1

2.9

Q3-3?

982

90

42.1

1.6

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Q4 peaks constrained

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2g

FWHM

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5 mol% Cs2O glass

Shape

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1

Position

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v-Silica

Peak

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Figure

10 mol% Cs2O glass

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Q4 unconstrained

10 mol% Cs2O glass

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Q4 peaks constrained by area

3a

1 - % Lor. indicates percentage Lorentzian

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ACCEPTED MANUSCRIPT Table 2: Fit parameters for 15, 20, 25 and 30 mol% Cs2O glasses Figure

Peak

Position

Shape

FWHM

Area

cm-1

%Lor.1

cm-1

%

Q4-1

1154

0

79.1

42.8

Q4 unconstrained

Q3-1

1095

85

45.7

54

Q3-2

1010

85

45.7

2

Q3-3?

968

85

45.7

Q4-1

1167

0

72.4

22.3

Q4 peaks constrained

Q4-2

1140

0

71.1

20.3

by area

Q3-1

1095

85

45.7

54.2

Q3-2

1010

85

45.7

2.0

Q3-3?

966

85

45.7

1.2

Q4-1

1150

0

62.7

20.1

1141

0

80.6

18.3

1095

80

54.7

58.1

996

80

54.7

2.1

Q2

943

80

54.7

1.4

Q4-1

1147

0

68.7

37.0

Q3-1

1095

80

54.7

59.6

Q3-2

995

80

54.7

2.1

Q2

942

80

54.7

1.3

Q4-1

1138

0

57.3

26.2

Q3-1

1097

80

55.7

67.3

Q3-2

1048

80

55.7

2.6

Q3-3

985

80

55.7

2.3

Q2

934

80

44.2

1.6

Q4-1

1135

0

39.5

8.0

Q3-1

1099

80

52.1

77.9

20 mol% Cs2O glass

3c

Q4-2

by area

Q3-1

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Q4 peaks constrained

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Q3-2

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Q4 unconstrained

3d

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20 mol% Cs2O glass

3e

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25 mol% Cs2O glass Q4 unconstrained

30 mol% Cs2O glass Q4 unconstrained

3f

35

1.2

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3b

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15 mol% Cs2O glass

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15 mol% Cs2O glass

ACCEPTED MANUSCRIPT 1041

80

52.1

7.0

Q3-3

992

80

52.1

4.2

Q2

931

80

43

2.9

30 mol% Cs2O glass

Q4-1

1131

0

51.0

14.8

Q4 unconstrained

Q3-1

1098

75

50.1

69.0

Q3-2

1041

75

50.1

8.1

Q3-3

992

75

50.1

Q2

931

75

52.2

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4.6

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1 - % Lor. indicates percentage Lorentzian

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Q3-2

3.6

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6